Vehicle manufacturers continually strive to improve fuel economy and reduce emissions while meeting customer expectations for performance and drivability. The availability of increasingly more powerful powertrain control computers has enabled more precise control of the vehicle powertrain and more efficient use of available energy to power the vehicle and related accessories.
For vehicle powertrains that include an internal combustion engine, either alone or in combination with other power sources (such as in a hybrid vehicle), accurate control of the combustion process is desirable to achieve exhaust emissions and fuel economy goals. As such, to maintain stable combustion under the varying operating conditions, accurate control of engine air and fuel supply is required. For example, air and spark adjustments may be performed in vehicles operating at low-loads, via closed-loop engine speed control, to maintain stable combustion. As such, the combustion stability limit may be defined in terms of an airflow, or a corresponding torque limit. Thus, a minimum engine torque may be set to ensure stable combustion. Such limits help to ensure that the vehicle will not operate in the unstable combustion region, despite variability due to throttle position, engine friction losses, fuel changes, age and wear of engine components, ambient operating conditions, etc.
Various closed-loop or feedback control based approaches may be used to lower vehicle exhaust emissions and improve fuel economy. One example approach for using closed-loop torque is shown by Gartner et al. is U.S. Pat. No. 7,275,518. Therein, a torque difference is adjusted using an engine speed-based torque offset calculation, while also limiting the torque difference by a maximum-engine-torque-available parameter and a minimum-engine-torque-available parameter. As a result, the engine may be operated closer to the minimum torque under varying operating conditions.
However, the inventors herein have identified potential issues with such an approach. As one example, no feedback control of torque may be provided when the requested engine torque is below the minimum torque required for combustion stability. Specifically, as a result of clipping torque at a minimum torque, more or less torque than is required may be provided under some conditions. Consequently, fuel may be wasted during those conditions. Further, the minimum torque includes significant spark retard, thus degrading fuel economy.
The above issues may be at least partly addressed by a method for controlling a vehicle torque, comprising during non-idle speed engine combustion operation, transitioning from air adjustment to spark adjustment to maintain actual engine torque at a desired engine torque when operating at a minimum aircharge for combustion stability. In this way, the desired torque may be accurately provided even when the vehicle is operated at or below minimum load.
In one example, during a tip-out event, torque may be reduced in accordance with a desired torque reduction profile. Therein, when the torque is above the minimum torque (that is, the minimum engine torque required to ensure stable combustion), torque control is achieved using at least airflow control. For example, airflow may be reduced while maintaining an amount of spark retard. Then, when the torque is below the minimum torque, further torque control is achieved using spark control. For example, spark may be retarded from MBT while maintaining the airflow at the minimum aircharge level.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.